1. Field of the Invention
This invention is related to the structure of a gearbox for all terrain vehicles, and in particular to one provided on the driven axle with a torque release mechanism for reducing the momentary increased loading and torque thereby preventing the chain or the driven toothed disc from being damaged.
2. Description of the Prior Art
Referring to FIGS. 1 and 2, the conventional all terrain vehicle 1 according to the present invention includes a frame 11, a steering mechanism 12, front wheel 13, a seat 14, rear wheels 15, and a power unit 2. The front portion of the frame 11 is pivotally connected with the steering mechanism 12 under which are mounted the front wheels 13. The seat 14 is arranged behind the steering mechanism 12 and on the frame 11. The power unit 2 is installed under the seat 14. As shown in FIG. 2, the power unit 2 includes an engine 21, a transmission case 22 and a gearbox 4 driven by the engine 21. The gearbox 4 has a final axle 235 which drives the rear wheel axle 24 via a chain 236. The rear wheels 15 are drivingly connected with the rear wheel axle 24.
As shown in FIG. 2, the fuel is first mixed with fresh air and then transmitted to a combustion chamber 211 of the engine 21 to generate power thereby causing a piston 212 to reciprocate and therefore rotating a crankshaft 213. Then, the crankshaft 213 will drive the transmission case 22.
The transmission case 22 includes a belt transmission 3 which includes a movable disc 31 mounted on the crankshaft 213, a driving disc 32 arranged on the crankshaft and mounted on one side of the movable disc 31, an inclined plate 33 mounted on the crankshaft 213 and arranged the other side of the movable disc 31, a plurality of rolling balls 34 fitted between the movable disc 31 and the inclined plate 33, a driven shaft 35 for power transmission, a driven disc 36 arranged on the driven shaft 35, a clutch 37 mounted on the driven shaft 35, and a belt 38. The movable disc 31 and the driving disc 32 form a driving disc 3a of a belt transmission mechanism 3. The belt 38 has an end arranged on the driving disc 3a and another end disposed on the driven disc 36. The gearbox 4 is provided with a shifting mechanism 23 having a shifting hub 232 sleeved on an axle 231. The shifting hub 232 is kept in place by two compressed springs 2311 and 2312 at two ends, so that the shift hub 232 can be moved along the axle 231. When the vehicle goes forward by shifting a fork 233 towards the forward position 2321, the shifting hub 232 will be forced to move to the right side thereby compressing the spring 2312 and therefore enabling the fork 233 to move smoothly to the forward position 2321 on the shifting hub 232. In the meantime, the fork 233 will push the slider 2341 on the main axle 234 to go smoothly into the hole 23421 of the forward gear 2342. Similarly, when the vehicle is shifted to go backward, the fork 233 will be moved to the backward position 2322 thereby forcing the shifting hub 232 to move to the left side and therefore compressing the spring 2311. As a result, the fork 233 will be smoothly moved to the backward position 2322 of the shifting hub 232. At the same time, the fork 233 will push the slider 2341 to move smoothly into the hole 23431 of the backward gear 2342 thus providing a smooth shifting operation. Hence, the forward gear 2342 will drive the main axle 234 which will in turn drive the driven gear 2351 thereby rotating the main axle 234. Thereafter, the main axle 234 will drive the gear 2351 to rotate the final axle 235 simultaneously. As a consequence, the driving gear 2352 at the outer side of the final axle 235 will drive the rear wheel 15 via a chain 236 thereby enabling the vehicle to go forward or backward.
The engine 21 utilizes the power generated from the explosion to reciprocate the piston 212 thereby rotating the crankshaft 213. The driving disc 3a of the belt transmission mechanism 3 will be rotated in unison with the crankshaft 213 thereby rotating the driven disc 36 to drive the driven shaft 35 via the clutch 37. The gear shifting mechanism 23 will drive the final axle 235 which will in turn drive a driving toothed disc 241 on the rear wheel axle 241 via the chain 236 thus turning the rear wheels 15 to cause the all terrain vehicle to travel.
In order for the terrain vehicle 1 to accelerate forward or upwards at the moment of impact as it touches ground, the accelerator must be fully pressed to further increase the performance of the output torque of the engine. Due to the gravitational force and the function of the shock absorber, the chain 236 between the sprocket 2352 and the driven disc 241 will be in a tensioned condition. Furthermore, the engine will output larger power and torque when the accelerator is pressed, the chain 236 and the driven disc 241 will be subject to a large torque thus probably pulling the chain 236 away the driven disc 241. As a consequence, the chain 236 and the driven disc 241 will even be broken or damaged thereby seriously influencing the safety of the all terrain vehicle.
Therefore, it is an object of the present invention to provide an improvement in the structure of a gearbox for all terrain vehicles which can reduce the large torque or loading generated in a transient moment so as to prevent the chain 236 and the driven disc 241 from being damaged.